Automatic keypad backlight adjustment on a mobile handheld electronic device

ABSTRACT

A method is set forth for automatically adjusting keypad brightness on a mobile electronic device having a light sensor, a keypad and a backlight for said keypad, comprising obtaining light level samples from the light sensor, turning the backlight from off to on in the event a current light level sample is less than a threshold value below which backlight illumination becomes necessary in order to read the keypad, and from off to on after a predetermined time period in the event the current light level sample is greater than the threshold value.

FIELD

The present application relates generally to electronic devices and moreparticularly to a method for controlling a backlight on a mobileelectronic device for enhanced keypad readability in ambient lightingconditions that give rise to contrast inversion.

BACKGROUND

Keypad brightness on a handheld electronic device may be adjusted fordifferent operating environments in order to enhance the visibility ofkey indicia (e.g. alphabetic characters, numbers). Conventional backlitkeypads or keyboards, such as Logitech Illuminated Keyboard part number920-000914 or MacBook Air part number MC233LL/A, are illuminated with amonochrome light source. A backlight controller varies the intensity ofthe monochrome light source based on measurements of ambient lightintensity. Alternatively, if no light sensor is provided it is customaryto illuminate the backlight whenever the device is powered on, to ensurethat the keypad is readable in the dark.

Typically, the keys of a conventional backlit keypad are opaque, forexample black with key indicia or characters thereon that aretransmissive in order to allow light to pass through for the purpose ofbacklighting in dark environments and that are white or some othercolour that creates suitable contrast for legibility. The backlightcontroller activates the keypad backlight when the keypad is exposed todim or moderately-intense ambient light, and deactivates the keypadbacklight when the keyboard is exposed to bright ambient light (e.g. inoutdoor or sunlight conditions).

Cathey (U.S. Pat. No. 7,129,930) describes a backlit keyboard in whicheach key includes dark-coloured key indicia printed on a transparentlight-coloured key face. Faraqe (U.S. Pat. No. 6,797,902) describes akeyboard in which each key is coupled to a large lamp that illuminatesthe transparent key face, and a small lamp that illuminates thedark-coloured key indicia. Knox (U.S. Pat. No. 6,950,087) describes abacklit keyboard in which the keys are illuminated by avariable-position prism that allows the user to manually adjust thecolour of the backlight. Stokic (US 2006/0061542) describes a keyboardthat is coupled to a display controller. The display controllertransmits key indicia to the keys, and adjusts the intensity of the keyindicia, and the contrast between the background and the key indicia,based on the ambient light level. Kreek (US 2007/0097065), the contentsof which are incorporated herein by reference, sets forth a method forautomatically adjusting screen and keypad brightness of non-contrastinversion keypads to improve legibility of the keypad on a mobilecommunication device, whereby in the event the ambient light levelexceeds 50 Lux for 30 seconds the keypad backlight switches from on tooff, and in the event the ambient light level falls below 16 Lux thekeypad backlight switches from off to on. Sellers (US 2007/0068784) setsforth a keyboard with lightpipes directing light to the underside of thekeys.

A problem of readability of characters on a keypad or keyboard (or otherinput device, such as an instrument panel) occurs for a certain range ofambient lighting conditions where contrast inversion occurs (i.e. when acharacter changes from being a dark element in high ambient lightconditions to being a light element in low ambient light conditions).This range of ambient lighting conditions occurs when there is ‘dim’lighting (i.e. between daylight and darkness) such that the contrastratio of the characters on the keypad decreases below a value at whichthey are no longer legible.

BRIEF DESCRIPTION OF THE DRAWINGS

The exemplary method for automatically controlling a backlight will bebetter understood with reference to the following description and to theFigures, in which:

FIG. 1 is a representation of a mobile handheld electronic device inconnection with which a method for automatically controlling a backlightis set forth in accordance with one embodiment;

FIG. 2 is a block diagram of certain internal components within theelectronic device of FIG. 1;

FIG. 3 is a graph showing contrast ratio vs. ambient light; and

FIG. 4 is a flowchart showing steps in a method for automaticallycontrolling a backlight in the electronic device of FIG. 1.

DETAILED DESCRIPTION OF THE EMBODIMENTS

According to an aspect of this specification a method is set forth forautomatically adjusting keypad brightness on a mobile electronic devicehaving a light sensor, keypad and a backlight for said keypad,comprising obtaining light level samples from the light sensor, turningthe backlight from off to on in the event a current light level sampleis less than a threshold value below which backlight illuminationbecomes necessary in order to read the keypad with an optimum contrastratio between light and dark regions of key indicia thereof, and from onto off after a predetermined time period in the event the current lightlevel sample is less than said threshold value but above a minimumcontrast ratio between said light and dark regions of key indicia.

As described in greater detail below, the foregoing aspects minimize theimpact on contrast ratio of the characters on a keypad in ‘dim’ lightingconditions by turning on the backlight such that the contrast betweenthe illuminated characters and the light reflected from surroundingmaterial maintains a ratio that is suitable for legibility (typicallybelow an optimum value of approximately 3:1 but above a minimum value of2:1).

Referring to FIG. 1, a mobile handheld electronic device is indicatedgenerally by the numeral 10. In the present embodiment, the electronicdevice 10 is based on the computing environment and functionality of amobile or smart phone. It will be understood, however, that theelectronic device 10 is not limited to mobile phones and smart phones.Other electronic devices are possible, such as a wireless personaldigital assistant, desktop computers, GPS receivers, and laptopcomputers. Referring again to the present embodiment, the electronicdevice 10 includes a housing 12 that frames an LCD display 31, a speaker33, a message notification indicator 18, a multi-directional device suchas a trackball 25, buttons 29 and a keyboard or keypad 24. Preferably,the message notification indicator 18 is in the form of a light pipehaving two internal branches terminating respectively in a LightEmitting Diode (LED) and an ambient light sensor, as set forth in U.S.Pat. No. 7,352,930 entitled SHARED LIGHT PIPE FOR A MESSAGE INDICATORAND LIGHT SENSOR, the contents of which are incorporated herein byreference. The housing 12 is made from a suitable material as will occurto those skilled in the art, and can be stored, for example, in aholster (not shown) that includes an attachment for attaching to auser's belt.

FIG. 2 is a block diagram showing certain components within an exemplaryembodiment of the portable electronic device 10. The portable electronicdevice 10 includes a processor 20 connected to a read-only-memory (ROM)21 that contains a plurality of applications executable by the processor20 for enabling the portable electronic device 10 to perform certainfunctions including, for example, Personal Identification Number (PIN)message functions, Short Message Service (SMS) message functions,address book and calendaring functions, camera functions, and cellulartelephone functions. More particularly, processor 20 may executeapplications within ROM 21 for notifying the user of events such asincoming calls and/or emails, appointments, tasks, etc. The processor 20is also connected to a random access memory unit (RAM) 22 and apersistent storage device 23 to facilitate various non-volatile storagefunctions of the portable electronic device 10. The processor 20receives input from one or more input devices, including keypad 24,trackball 25, ambient light sensor 36 (preferably disposed within thehousing 12 and receiving light from a light pipe terminating atindicator 18), and user buttons 29.

The processor 20 outputs to one or more output devices, including aLiquid Crystal Display (LCD) 31, a backlight controller 26 and messagenotification indicator 18. A microphone 32 and phone speaker 33 areconnected to the processor 20 for cellular telephone functions. Theprocessor 20 is also connected to a modem and radio device 34. The modemand radio device 34 is used to connect to wireless networks and transmitand receive voice and data communications through an antenna 35.

A typical backlight system comprises a backlight lighting source 37,such as a series of LEDs or a lamp located behind the keypad 24, andbacklight controller 26 to control activation of the backlight 37. Thelighting source may be fluorescent, incandescent, electroluminescent orany other suitable lighting source. As the backlight 37 is illuminated,light shines through the keypad 24 providing backlight to the keyindicia thereon. The intensity of the backlight level may be controlledby the controller 26 in any of a plurality of ways, such as by adjustingcurrent or voltage applied to the lighting source, by selectivelyactivating a selected number of lighting sources (e.g. one, several orall LEDs) or by selectively controlling the activation duty cycle of theactivated lighting sources (e.g. a duty cycle anywhere between 0% to100% may be used).

To assist with one method of adjusting the backlight level, light sensor36 is provided within the device 10. Sensor 36 is preferably a lightsensitive device that converts detected light levels into an electricalsignal, such as a voltage. The sensor 36 may be located anywhere on orwithin the device 10, having considerations for aesthetics and operationcharacteristics of sensor 36. However as discussed above, in oneembodiment, an opening for light to be received by sensor 36 is locatedon the front cover of the housing of device 10 (to reduce thepossibility of blockage of the opening), which accommodates a Y-shapedlight guide that terminates at one end at message indicator 18 andbranches inside the housing 12 so as to terminate in a message LED andthe light sensor 36. Multiple sensors 36 may be provided and softwaremay provide different emphasis on signals provided from differentsensors 36. The signal(s) provided by sensor(s) 36 can be used by acircuit in device 10 to determine when device 10 is in a well-lit, dimlylit or moderately-lit environment, as discussed in greater detail below.This information can then be used to control backlight levels for keypad24.

Reference is now made to FIG. 3 showing a graph of contrast ratio vs.ambient light. In order to ensure readability in the dark, backlight 37must be turned on for ambient light levels less than a threshold value(corresponding to an optimum contrast ratio (CR Optimum) of 3:1 in theexemplary embodiment). In FIG. 3, the threshold is approximately 18 Lux.In general, the threshold may be determined according to the followingequation:

Ethreshold=pi*Lbacklight/(3*Rkey−Rcharacter), where

Ethreshold is the optimum ambient light threshold (in Lux),Lbacklight is the average luminance of the backlight characters incd/m²,Rkey is the reflectance of the key surface, andRcharacter is the reflectance of the key indicia.

The foregoing equation is derived from the well known method ofconverting luminance measured in cd/m² to illuminance measured in Lux:E=pi*L/R.

However, as discussed above, a problem of readability of characters (orkey indicia) on keypad 24 occurs when backlight 37 is on in a range of‘dim’ lighting conditions below and above the ambient light level atwhich contrast inversion takes place (i.e. the range of ambient lightlevels below and above the vertical Contrast Inversion line and below aminimum contrast ratio (CR Minimum) at which the indicia is no longerlegible). Although legibility is somewhat subjective, it has been foundthat for the exemplary embodiment the CR Optimum value of approximately3:1 represents a threshold below which legibility may start to become aproblem and a value of approximately 2:1 represents the minimumthreshold (CR minimum) for legibility. Generally, the ambient lightlevel at which contrast inversion takes place may be calculated bysolving the following:

Einversion=pi*Lbacklight/(Rkey−Rcharacter).

The curve CR Backlight depicts the contrast ratio of the keypad 24 overa range of ambient light levels while the backlight 37 is illuminated.The line CR Reflected depicts the contrast ratio of the keypad 24 withthe backlight 37 extinguished. A person of skill in the art willappreciate that the values, lines and curves depicted in FIG. 3 relateto an exemplary keypad 24 and backlight 37, but that the values, linesand curves may vary depending on variables such as reflectance of paintused for the indicia on keys of the keypad 24, colour and intensity ofbacklight 37, etc.

From the foregoing, it will be appreciated that prior to turning off thebacklight, readability may suffer as a result of operating the backlightin the region of contrast inversion (e.g. the threshold ambient lightlevel of 50 Lux results in a contrast ratio less than CR Minimum). Thepresent disclosure sets forth a solution to his readability problem.

With reference to FIG. 4, a method is set forth for minimizing theimpact of reduced contrast ratio for ambient lighting conditions in thevicinity of contrast inversion. Upon starting the algorithm (step 50)when the device 10 is turned on, the backlight mode is normallyinitialized to an appropriate mode using the ambient lighting sensed bythe light sensor 36 at that time. Next, light sensor samples are takenat set intervals (step 52). The amount of time between each light sensorsample determines the sampling rate. A typical sampling rate is onesample per 1.2 seconds although in some situations the sampling rate maybe increased to 400 ms temporarily for 5 samples to facilitate quickadjustment of the keypad backlight 37. At step 56, a determination ismade as to whether the backlight is off 37 (the backlight will be offwhen the device 10 is initially powered on, or when the device isoperating in bright ambient lighting conditions).

In the event that the backlight is off (a “Yes” at step 56), then thelight sample is compared to a threshold T (step 58). As discussed above,in the exemplary embodiment the threshold T is preferably approximately18 Lux, which is the threshold below which backlight illuminationbecomes necessary in order to read the key indicia. If the light sampleis less than T, then backlight controller 26 turns on the backlight 37under control of processor 20 (step 60). The method then reverts to step52. If the light sample is not less than T, then the backlight 37remains off and the method reverts to step 52.

In the event that the backlight is on (e.g. due to operation in darkconditions), then the light sample is again compared to the threshold T(step 62). If the light sample continues to be less than T, then themethod reverts to step 52. However, if the light sample is not less thanT, then the backlight 37 is turned off (step 64) to avoid legibilityproblems resulting from backlight operation in the range of ambientlight conditions in the region of contrast inversion. However, in orderto avoid disruptive or distracting flickering of the backlight 37 whenthe ambient light is in the region of threshold T, a time delay isintroduced before turning off the backlight. According to the exemplaryembodiment, a 2 second time delay is provided although longer delays maybe necessary to filter out any momentary bright sample values. Also,rather than abruptly switching off the backlight 37, it is preferablethat backlight controller 26 turn off the backlight 37 smoothly butquickly, for example by adjusting current or voltage applied to thelighting source, by selectively activating a selected number of lightingsources (e.g. one, several or all LEDs) or by selectively controllingthe activation duty cycle of the activated lighting sources (e.g. a dutycycle anywhere between 0% to 100% may be used). Moreover, the backlight37 is preferably not turned off until the light sample value is slightlyhigher (e.g. 24 Lux in the exemplary embodiment) than the threshold T atstep 62, but not so high that the contrast ratio falls below the minimumcontrast ratio (CR Minimum) between light and dark regions of keyindicia (e.g. CR Minimum of 2:1, which corresponds to approximately 30Lux in the exemplary embodiment). This difference in backlight switchingthresholds results in a hysteresis loop comprising steps 62 and 63 thateffectively shifts the contrast inversion point to a safer (i.e. higher)level before switching off the backlight 37, thereby avoiding backlightflicker (i.e. transitioning between the on and off states continuouslywhen the light sample is close to the threshold value T of step 62). Inparticular, according to the exemplary embodiment of FIG. 3, the lightsample value of 24 Lux is greater than the threshold T=18 Lux (step 62)but less than the value that corresponds to the minimum contrast ratio(step 63). In the exemplary embodiment, CR Minimum=2:1, whichcorresponds to approximately 30 Lux.

By operating the backlight 37 in accordance with the exemplary method ofFIG. 4, the contrast between the illuminated characters (key indicia)and the light reflected from the surrounding material of the keypad 24maintains a ratio that is suitable for legibility (i.e. in accordancewith the curve CR Optimum in FIG. 3). A person of skill in the art willappreciate that the light sensor 36 must be sufficiently sensitive toresolve subtle changes in light level, especially in the 1 to 100 Luxrange.

While the embodiments described herein are directed to particularimplementations of the method for automatically adjusting keyboardbrightness on a mobile handheld electronic device, it will be understoodthat modifications and variations to these embodiments are within thescope and sphere of the present application. For example, thebacklighting brightness adjustment methodology set forth herein is notlimited in its application to handheld electronic devices but mayadvantageously applied to other electronic devices such as desktopcomputers, cellular telephones, GPS receivers, smart telephones, andlaptop computers. Also, as discussed above, specific values for thethreshold T may vary depending on factors such as reflectance of thepaint used for the key indicia, etc. Many other modifications andvariations may occur to those skilled in the art. All such modificationsand variations are believed to be within the sphere and scope of thepresent application.

1. A method for automatically adjusting keypad brightness on a mobileelectronic device having a light sensor, a keypad and a backlight forsaid keypad, comprising: obtaining light level samples from said lightsensor; and turning the backlight from off to on in the event a currentlight level sample is less than a threshold value below which backlightillumination becomes necessary in order to read the keypad with anoptimum contrast ratio between light and dark regions of key indiciathereof, and from on to off in the event the current light level sampleis less than said threshold value but above a minimum contrast ratiobetween said light and dark regions of key indicia.
 2. The method ofclaim 1, wherein said threshold value ispi*Lbacklight/(3*Rkey−Rcharacter), where Lbacklight is the averageluminance of said key indicia in cd/m², Rkey is the reflectance of saidkeypad, and Rcharacter is the reflectance of said key indicia.
 3. Themethod of claim 2, wherein said threshold value is approximately 18 Lux.4. The method of claim 1, wherein said backlight is turned from on tooff after a predetermined time period.
 5. The method of claim 4, whereinsaid time period is approximately 2 seconds.
 6. The method of claim 1,wherein said optimum contrast ratio is approximately 3:1.
 7. The methodof claim 1, wherein said minimum contrast ratio is approximately 2:1. 8.The method of claim 1, wherein said light level samples are obtained ata predetermined sampling rate.
 9. The method of claim 8, wherein saidpredetermined sampling rate is one sample per approximately 1.2 seconds.10. The method of claim 9, wherein said predetermined sampling rate istemporarily increased to one sample per approximately 400 ms tofacilitate quick adjustment of said backlight.
 11. A mobile device,comprising: a light sensor; a keypad; a backlight for said keypad; aprocessor for obtaining light level samples from said light sensor andcomparing said light level samples with a threshold value and a minimumcontrast ratio; and a backlight controller for turning the backlightfrom off to on in the event a current light level sample is less thansaid threshold value below which backlight illumination becomesnecessary in order to read the keypad with an optimum contrast ratiobetween light and dark regions of key indicia thereof, and from on tooff in the event the current light level sample is less than saidthreshold value but above said minimum contrast ratio between said lightand dark regions of key indicia.
 12. The mobile device of claim 11,wherein said threshold value is approximately 18 Lux.
 13. The mobiledevice of claim 11, wherein said optimum contrast ratio is approximately3:1.
 14. The mobile device of claim 11, wherein said minimum contrastratio is approximately 2:1.